1. Field of the Invention
This invention relates to heavy duty pneumatic tires, and more particularly to a heavy duty pneumatic tire having improved traction and braking performances on a wet road surface or wet performances without damaging resistance to uneven wear and resistance to stone biting.
2. Related Art Statement
In the pneumatic tire having lug grooves separated from each other in the circumferential direction of the tire and extending substantially in the widthwise direction of the tire and island portions defined by such lug grooves or a lug pattern, traction force and braking force are excellent. Such tires are widely used as a tire for truck and bus, a tire for a construction vehicle and the like.
On the other hand, there is a chance of a truck, bus and the like provided with such a tire running at a high speed according to road conditions, hence, it is highly demanded to have steering property and stability in such tires.
In order that such a demand is satisfied by the conventional technique, as shown in FIG. 17a, lug grooves 14 are formed in each of half regions in widthwise direction of a tread portion 12 of a tire 10 at a shifting of about half pitch in the circumferential direction. Further, a part of the lug groove 14 substantially extending from a shoulder portion 16 toward a central region of the tread portion 12 in the widthwise direction is bent toward the circumferential direction of the tire. Then, the lug grooves 14, 14 formed in the respective half regions of the tread portion 12 are connected to each other through a subsidiary groove 18 slanting and crossed with respect to the equator P of the tire. The bent portions of the two lug grooves 14 formed in each half region of the tread portion and separated from each other in the circumferential direction of the tire are connected to each other through another subsidiary groove 20 slanting and crossed with respect to a plane separated in parallel to the equatorial plane of the tire consequently an island portion 22 is defined by these subsidiary grooves 18 and 20.
In such a conventional tire 10, the edge of the island portion 22 slants and is crossed with the equatorial plane or the plane separated in parallel to the equatorial plane, so that not only the traction performance and braking performance are improved but also the lateral slipping of the tire against the movement in the widthwise direction of the tire is suppressed to contribute the improvement of the steering property and stability of the tire. Furthermore, when the tire is run on a wet road surface, the above edge develops a so-called edge effect that the edge cuts a water film formed on the road surface to contact with the road surface, so that the wet performances are improved.
As shown in FIG. 17b, the depths of the subsidiary grooves 18 and 20 from the tread surface are about 30% of the depth of the lug groove 14 to ensure the rigidity of the island portion 22 defined by these subsidiary grooves 18 and 20 at the kicking-out side edge. Thus the uneven wear at the kicking-out side region of the island portion 22 during the running of the tire is prevented.
However, the depths of the subsidiary grooves 18 and 20 formed in the central region of the tread portion 12 are shallower than that of the lug groove 14, so that the drainage performance of the tire is degraded as the wearing of the tread surface promotes during the running and hence the given object can not be achieved.
Furthermore, once wear is caused at the edge of the island portion 22 in the central region of the tire tread, the uneven wear gradually proceeds due to the difference in the size of the tire, and also the edge is worn, so that the edge effect can not be expected and consequently the steering property and stability on wet road surface are degraded.